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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN">
<html>
<head>
<title>Xapian: Remote Backend Protocol</title>
</head>
<body bgcolor="white" text="black">
<h1>Remote Backend Protocol</h1>
<p>
This document describes <em>version 35.0</em> of the protocol used by Xapian's
remote backend. The major protocol version increased to 35 in Xapian 1.1.5.
Clients and servers must support matching major protocol versions and the
client's minor protocol version must be the same or lower. This means that for
a minor protocol version change, you can upgrade first servers and then clients
and everything should work during the upgrades.
</p>
<p>
The protocol assumes a reliable two-way connection across which
arbitrary data can be sent - this could be provided by a TCP socket for example
(as it is with xapian-tcpsrv), but any such connection could be used. For
example, you could used xapian-progsrv across an ssh connection, or even
a custom server across a suitable serial connection.
</p>
<p>
All messages start with a single byte identifying code. A message from client
to server has a <code>MSG_XXX</code> identifying code, while a message from
server to client has a <code>REPLY_XXX</code> identifying code (but note that a
reply might not actually be in response to a message - REPLY_GREETING isn't -
and some messages result in multiple replies).
</p>
<p>
The identifying code is followed by the encoded length of the contents followed
by the contents themselves.
</p>
<p>
Inside the contents, strings are generally passed as an encoded
length followed by the string data (this is indicated below by
<code>L<...></code>) except when the string is the last or only
thing in the contents in which case we know the length because
we know the length of the contents so we don't need to explicitly
specify it.
</p>
<p>
Integers are encoded using the same encoding used for string lengths
(indicated by <code>I<...></code> below).
</p>
<p>
Floating pointing values are passed using a bit packed encoding of the sign
and exponent and a base-256 encoding of the mantissa which avoids any rounding
issues (assuming that both machines have FLT_RADIX equal to some power of 2).
This is indicated by <code>F<...></code> below.
</p>
<p>
Boolean values are passed as a single byte which is the ASCII character
value for <code>0</code> or <code>1</code>. This is indicated by
<code>B<...></code> below.
</p>
<h2>Server greeting and statistics</h2>
<ul>
<li> <code>REPLY_GREETING <protocol major version> <protocol minor version> I<db doc count> I<last docid> B<has positions?> I<db total length> <UUID></code>
</ul>
<p>
The protocol major and minor versions are passed as a single byte each (e.g.
<code>'\x1e\x01'</code> for version 30.1). The server and client must
understand the same protocol major version, and the server protocol minor
version must be greater than or equal to that of the client (this means that
the server understands newer MSG_<i>XXX</i>, but will only send newer
REPLY_<i>YYY</i> in response to an appropriate client message.
</p>
<h2>Exception</h2>
<ul>
<li> <code>REPLY_EXCEPTION <serialised Xapian::Error object></code>
</ul>
<p>
If an unknown exception is caught by the server, this message is sent but
with empty contents.
</p>
<p>
This message can be sent at any point - the serialised exception is
unserialised by the client and thrown. The server and client both
abort any current sequence of messages.
</p>
<h2>Write Access</h2>
<ul>
<li> <code>MSG_WRITEACCESS</code>
<li> <code>REPLY_UPDATE I<db doc count> I<last docid> B<has positions?> I<db total length> <UUID></code>
</ul>
<p>
The reply is the same as for <code>MSG_UPDATE</code>. If write access isn't
supported or the database is locked by another writer, then an exception is
thrown.
</p>
<p>
By default the server is also read-only, even if writing is supported.
If the client wants to be able to write, it needs to request this
explicitly. We do this so that the same server can support multiple
read-only clients and one writing client at once, without the protocol
for read-only clients requiring an extra message. The overhead of an
extra message exchange for a writer is likely to matter as indexing
is rarely so real-time critical as searching.
</p>
<h2>All Terms</h2>
<ul>
<li> <code>MSG_ALLTERMS</code>
<li> <code>REPLY_ALLTERMS I<term freq> L<term name></code>
<li> <code>...</code>
<li> <code>REPLY_DONE</code>
</ul>
<h2>Term Exists</h2>
<ul>
<li> <code>MSG_TERMEXISTS <term name></code>
<li> <code>REPLY_TERMEXISTS</code> or <code>REPLY_TERMDOESNTEXIST</code>
</ul>
<h2>Term Frequency</h2>
<ul>
<li> <code>MSG_TERMFREQ <term name></code>
<li> <code>REPLY_TERMFREQ I<term freq></code>
</ul>
<h2>Collection Frequency</h2>
<ul>
<li> <code>MSG_COLLFREQ <term name></code>
<li> <code>REPLY_COLLFREQ I<collection freq></code>
</ul>
<h2>Document</h2>
<ul>
<li> <code>MSG_DOCUMENT I<document id></code>
<li> <code>REPLY_DOCDATA L<document data></code>
<li> <code>REPLY_VALUE I<value no> <value></code>
<li> <code>...</code>
<li> <code>REPLY_DONE</code>
</ul>
<h2>Document Length</h2>
<ul>
<li> <code>MSG_DOCLENGTH I<document id></code>
<li> <code>REPLY_DOCLENGTH I<document length></code>
</ul>
<h2>Keep Alive</h2>
<ul>
<li> <code>MSG_KEEPALIVE</code>
<li> <code>REPLY_DONE</code>
</ul>
<h2>Reopen</h2>
<ul>
<li> <code>MSG_REOPEN</code>
<li> <code>REPLY_UPDATE I<db doc count> I<last docid> B<has positions?> I<db total length> <UUID></code>
</ul>
<p>The reply is the same as for <code>MSG_UPDATE</code>.</p>
<h2>Query</h2>
<ul>
<li> <code>MSG_QUERY L<serialised Xapian::Query object>
I<query length>
I<collapse max> [I<collapse key number> (if collapse_max non-zero)]
<docid order>
I<sort key number> <sort by> B<sort value forward>
<percent cutoff> F<weight cutoff> <serialised Xapian::Weight object> <serialised Xapian::RSet object> [L<serialised Xapian::MatchSpy object>...]
</code>
<li> <code>REPLY_STATS <serialised Stats object></code>
<li> <code>MSG_GETMSET I<first> I<max items> I<check at least>
<serialised global Stats object></code>
<li> <code>REPLY_RESULTS
L<the result of calling serialise_results() on each Xapian::MatchSpy>
<serialised Xapian::MSet object></code>
</ul>
<p>docid order is <code>'0'</code>, <code>'1'</code> or <code>'2'</code>.</p>
<p>sort by is <code>'0'</code>, <code>'1'</code>, <code>'2'</code> or <code>'3'</code>.</p>
<h2>Termlist</h2>
<ul>
<li> <code>MSG_TERMLIST I<document id></code>
<li> <code>REPLY_DOCLENGTH I<document length></code>
<li> <code>REPLY_TERMLIST I<wdf> I<term freq> L<term name></code>
<li> <code>...</code>
<li> <code>REPLY_DONE</code>
</ul>
<h2>Positionlist</h2>
<ul>
<li> <code>MSG_POSITIONLIST I<document id> <term name></code>
<li> <code>REPLY_POSITIONLIST I<termpos delta - 1></code>
<li> <code>...</code>
<li> <code>REPLY_DONE</code>
</ul>
<p>
Since positions must be strictly monotonically increasing, we encode
<tt>(pos - lastpos - 1)</tt> so that small differences
between large position values can still be encoded compactly. The first
position is encoded as its true value.
</p>
<h2>Postlist</h2>
<ul>
<li> <code>MSG_POSTLIST <term name></code>
<li> <code>REPLY_POSTLISTSTART I<termfreq> I<collfreq></code>
<li> <code>REPLY_POSTLISTITEM I<docid delta - 1> I<wdf> F<document length></code>
<li> <code>...</code>
<li> <code>REPLY_DONE</code>
</ul>
<p>
Since document IDs in postlists must be strictly monotonically increasing, we
encode <tt>(docid - lastdocid - 1)</tt> so that small
differences between large document IDs can still be encoded compactly. The
first document ID is encoded as its true value - 1 (since document IDs are always > 0).
</p>
<h2>Shut Down</h2>
<ul>
<li> <code>MSG_SHUTDOWN</code>
</ul>
<p>
No reply is sent - this message signals that the client has ended the session.
</p>
<h2>Update</h2>
<ul>
<li> <code>MSG_UPDATE</code>
<li> <code>REPLY_UPDATE I<db doc count> I<last docid> B<has positions?> I<db total length> <UUID></code>
</ul>
<p>
Only useful for a <code>WritableDatabase</code> (since the same statistics
are sent when the connection is initiated in the <code>REPLY_GREETING</code>
and they don't change if the database can't change).
</p>
<h2>Add document</h2>
<ul>
<li> <code>MSG_ADDDOCUMENT <serialised Xapian::Document object></code>
<li> <code>REPLY_ADDDOCUMENT I<document id></code>
</ul>
<h2>Delete document</h2>
<ul>
<li> <code>MSG_DELETEDOCUMENT I<document id></code>
<li> <code>REPLY_DONE</code>
</ul>
<h2>Delete document by term</h2>
<ul>
<li> <code>MSG_DELETEDOCUMENTTERM <term name></code>
</ul>
<h2>Replace document</h2>
<ul>
<li> <code>MSG_REPLACEDOCUMENT I<document id> <serialised Xapian::Document object></code>
</ul>
<h2>Replace document by term</h2>
<ul>
<li> <code>MSG_REPLACEDOCUMENTTERM L<term name> <serialised Xapian::Document object></code>
</ul>
<h2>Cancel</h2>
<ul>
<li> <code>MSG_CANCEL</code>
</ul>
<h2>Commit</h2>
<ul>
<li> <code>MSG_COMMIT</code>
<li> <code>REPLY_DONE</code>
</ul>
<h2>Set metadata</h2>
<ul>
<li> <code>MSG_SETMETADATA L<key> <value></code>
</ul>
<h2>Get metadata</h2>
<ul>
<li> <code>MSG_GETMETADATA <key></code>
<li> <code>REPLY_METADATA <value></code>
</ul>
<h2>Add spelling</h2>
<ul>
<li> <code>MSG_ADDSPELLING I<freqinc> <word></code>
</ul>
<h2>Remove spelling</h2>
<ul>
<li> <code>MSG_REMOVESPELLING I<freqdec> <word></code>
</ul>
</body>
</html>
|
